The invention relates generally to communications between a transponder and a transmitter-receiver antenna and more specifically to reducing the residual power remaining in a transmitter-receiver coil after transmitting an interrogation signal to the transponder.
In applications such as automotive burglar alarms, it is desirable to use a single coil to transmit a relatively high power interrogation signal to energize a transponder carried by a particular individual. The transponder lacks its own power source but uses the power in the interrogation signal to transmit a relatively low power response signal to the transmitter-receiver coil. The response signal contains information such as an identification code.
One problem associated with such a system is that residual power remains in the coil even after the coil is no longer driven with the interrogation signal. This remaining residual power takes time to decay from the transmitter/receiver coil. If the response signal arrives at the coil before the remaining residual power in the coil is adequately reduced, the response signal can be corrupted or undetectable.
U.S. Pat. No. 5,025,492 uses a damping circuit to reduce the power in the antenna. The circuit, comprised of a switch and several resistors, is adapted to be connected to the antenna after sending an interrogation signal and disconnected before receiving a response signal. However, the addition of several components and the increased board size increase the cost of the system. Another problem with using additional components is that heat is generated by the additional components which must be removed.